Mutarotation of a-D-glucose

A study of the rate of mutarotation of a-D-glucose in dimethyl-fonnamide in the presence of poly(methacrylic acid), PMA, using polyelectrol5de samples of a different type and degree of stereoregulaiity, has recently been reported (J5). The results of this study indicate that the rate constant for mutarotation of glucose is higher with isotactic PMA than with syndiotactic or conventional PMA. Conventional... 

The results obtained for the mutarotation of a-D-glucose in water or methanol-water, and with certain catalysts, are summarized in Table XII. 

The light-promoted mutarotation of a-D-glucose in DMSO in the presence of aromatic hydrocarbon sensitisers has been reported. It has been confirmed, by use of h.p.l.c., that the mutarotation of a- and -D-glucopyranose in dilute, neutral aqueous solution is a slow process, taking approximately three hours at ambient temperature to reach completion. The mutarotation rate of the j8-compound was 1.4 times that of the a-isomer, and the a fi ratio at equilibrium was 37.2 62.8. In addition, it was found that a-D-glucopyranose complexes more strongly with Ca ions then does jJ-D-glucopyranose. ... 

The oxidation of a-D-glucose occurs at less than 1% of the rate of oxidation of the /3 anomer. Because these two forms exist in solution in equilibrium in the proportion of 36% (a) and 64% (/3), mutarotation of the a to the /3 form must be allowed to reach equilibrium in the sample and standards for consistent... 

Certain procedures make it possible to obtain the a and 3 anomers of glucose in pure form. A 1-molar solution of a-D-glucose has a rotation value [a]o of +112°, while a corresponding solution of p-D-glucose has a value of +19°. These values change spontaneously, however, and after a certain time reach the same end point of +52°. The reason for this is that, in solution, mutarotation leads to an equilibrium between the a and p forms in which, independently of the starting conditions, 62% of the molecules are present in the P form and 38% in the a form. 

Increase of hydrostatic pressure is reported to increase the rate of mutarotation, and, at 2500 atm, the mutarotation of D-glucose is 3.4 times that at atmospheric pressure. From such data, Sander calculated that the decrease in volume of a-D-glucose in the activated state is in the range of 12.5 to 21.2 ml per mole. 

The structural lability of the cyclic hemiacetals is a consequence of their oxo-cyclo tautomerism, which allows a rapid interconversion of the (2R)-configurated enantiomer into the (2S)-enantiomer and vice versa via the ring-opened oxo form (Fig. (4)). An analogous and well investigated process is the formation of an equilibrium mixture of a-D-glucose and p-D-glucose on dissolution of the pure a- or P-isomer in water by mutarotation. 

Here it would be necessary to be sure that the rotational shifts arise through the coupling reactions and not through such secondary effects as mutarotation of the D-glucose or temperature changes. The rotational shifts are not large, but they could be obtained with good accuracy on a precision polarimeter. 

When D-glucose is heated at 150° in the absence of a catalyst, the distribution of disaccharide isomers obtained is equivalent to that of acid reversion. The mutarotation of molten D-glucose and of D-glucose below its melting point have also been studied. 

H. S. Isbell, A new crystalline calcium chloride compound of a-D-glucose and its rotation and mutarotation in aqueous solution, Bur. Standards J. Res., 5 (1930) 741-755. 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

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